The present invention relates to a controller of a variable displacement compressor.
The refrigeration circuit of a typical vehicle air conditioner includes a compressor, such as a variable displacement swash plate type compressor. A typical variable displacement swash plate type compressor includes a displacement control mechanism for maintaining the pressure at the outlet of an evaporator, which will be referred to as the suction pressure Ps, at a target value, which will be referred to as target suction pressure. The displacement control mechanism feedback controls the displacement of the compressor, or the inclination angle of the swash plate, by referring to the suction pressure Ps such that the displacement corresponds to the cooling load. A typical displacement mechanism includes a displacement control valve, which is called an internally controlled valve. The internally controlled valve detects the absolute value of the suction pressure Ps by means of a pressure sensitive member such as a bellows or a diaphragm. The internally controlled valve moves a valve body by the displacement of the pressure sensing member to adjust the valve opening size. Accordingly, the pressure in a swash plate chamber (a crank chamber), or the crank pressure Pc is changed, which changes the inclination of the swash plate. However, an internally controlled valve that has a simple structure and a single target suction pressure cannot respond to the changes in air conditioning demands. Therefore, control valves having a target suction pressure that can be changed by external electrical control are becoming standard.
A typical electrically controlled control valve is a combination of an internally controlled valve and an actuator such as an electromagnetic solenoid, which applies an electrically controlled force. Mechanical spring force, which acts on the pressure sensing member is externally controlled to change the target suction pressure. The target suction pressure is changed by controlling a current to the electromagnetic solenoid in an analog or a digital manner. The supplied current is controlled by a controller having a microcomputer that is designed for air conditioning. Specifically, the controller executes a proportional and integral (PI) control procedure or a proportional, integral and differential (PID) control procedure based on temperature information from a temperature sensor located near the evaporator or in a passenger compartment for continuously controlling the current. As a result, the compressor theoretically maintains an ideal displacement, or a displacement that corresponds to the magnitude of the cooling load.
However, to execute a PI control procedure or a PID control procedure for continuously and finely controlling the target suction pressure, the controller, which includes a microcomputer, must continuously receive temperature information from the temperature sensor and compute the current supplied to a control valve. Thus, the controller must have a high-performance microcomputer to bear a high computation load. Even if the controller has a high-performance microcomputer, the controller receives temperature data relatively frequently (at an extremely short cycle). Thus, the controller cannot be used for other purposes, which increases the ratio of cost of the controller in the total cost of the compressor.
In a displacement control procedure in which the absolute value of the suction pressure Ps is used as a reference, changing of the target suction pressure by electrical control does not always quickly change the actual suction pressure to the target suction pressure. This is because whether the actual suction pressure quickly seeks a target suction pressure when the target suction pressure is changed greatly depends on the absolute magnitude of the cooling load. Therefore, even if the target suction pressure is finely and continuously controlled by controlling the current to the control valve, changes in the compressor displacement are likely to be too slow or too sudden.
Accordingly, it is an objective of the present invention to provide a control device of a variable displacement compressor that has a simple structure and improves the controllability and response of displacement control.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a controller for a variable displacement compressor, which is used for air conditioning a compartment, is provided. The compressor includes a suction pressure zone, a discharge pressure zone, and a control chamber, which is connected to the suction pressure zone and to the discharge pressure zone. The pressure in the control chamber is adjusted for controlling the displacement of the compressor. The controller includes a refrigerant circuit, a control valve, a detection circuit and a pressure difference changer. The refrigerant circuit is connected to the compressor. Two pressure monitoring points are located in the refrigerant circuit. The control valve controls the pressure in the control chamber. The control valve operates based on the actual pressure difference between the pressure monitoring points such that a target value of the pressure difference between the pressure monitoring points, which is externally determined, is maintained. The detection circuit includes a temperature sensor for monitoring a temperature that represents the temperature of the compartment. The detection circuit produces a first detection signal when the sensed temperature exceeds a threshold value and a second detection signal when the sensed temperature falls below the threshold value. The pressure difference changer gradually increases the target value of the pressure difference when the first signal is received from the detection circuit and gradually decreases the target value of the pressure difference when the second signal is received from the detection circuit.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.